The prevalence of a disease plays an important role in your probability of having it given you test positive.
This is less relevant for testing positive for SARS-CoV-2 infection since RT-PCR rarely (if ever) has false positives. However, this may be relevant for antibody tests, which are less precise. Here is a quick explainer for how this works.
The numbers in this explainer are made up. Are some real numbers for the sensitivity and specificity (as it stands today): r tufte::margin_note("1. https://www.mayoclinicproceedings.org/article/S0025-6196(20)30365-7/fulltext <br> 2. https://www.fda.gov/media/136151/download <br> 3. https://www.centerforhealthsecurity.org/resources/COVID-19/serology/Serology-based-tests-for-COVID-19.html")
RT-PCR:
r emo::ji("point_right") sensitivity: can be as low as 70%, maybe closer to 90%¹
r emo::ji("point_right") specificity: ~100%²
Antibody test (depends on the test, the one approved in US³):
r emo::ji("point_right") sensitivity: 93.8%
r emo::ji("point_right") specificity: 95.6%